It is now accepted that the adult vertebrate brain fosters the birth and functional incorporation of newly formed neurons (Goldman and Nottebohm, 1984; Paton and Nottebohm, 1984; Burd and Nottebohm, 1985). However, it was long thought that no new neurons could be added to the adult mammalian brain. This dogma was challenged in the 1960's when autoradiographic evidence of new neuron formation in the hippocampal dentate gyrus, olfactory bulb, and cerebral cortex of the adult rat was presented (Altman, 1962, 1963; Altman and Das, 1965, 1966a,b). It is now accepted that within all mammalian species, including humans (Eriksson et al., 1998), there are two major reservoirs of neuronal stem cells, one located in the subgranular zone (SGZ) of the hippocampal dentate gyrus and another in the subventricular zone (SVZ) (Gross, 2000). Neural stem cells in the SVZ facilitate formation of new neurons that migrate rostrally to populate the olfactory bulb, while neural stem cells in the SGZ produce neurons that integrate locally in the granular layer of the dentate gyrus, a region of the hippocampus that exhibits lifelong structural and functional plasticity.
The process of new neuron formation in the adult mouse brain can be influenced by environmental, chemical and genetic variables. As demonstrated by Gage and colleagues, neurogenesis in the adult mouse brain is enhanced when animals are exposed to an enriched environment (Kempermann et al., 1998) or able to exercise voluntarily (van Praag et al., 1999). More recently, antidepressant drugs have been shown to enhance levels of adult neurogenesis in animals, including humans (Schmidt and Duman, 2007; Boldrini et al., 2009). Among many genes reported to impact adult neurogenesis is the gene encoding neuronal PAS domain protein 3 (NPAS3), a central nervous system (CNS)-specific transcription factor that has been associated with schizophrenia and bipolar disorder (Kamnsasaran et al., 2003; Pickard et al., 2005, 2006, 2009; Lavedan et al., 2008). Animals missing both copies of the NPAS3 gene suffer a profound loss of adult hippocampal neurogenesis coupled with significant behavioral deficits (Pieper et al., 2005). Knowing that impaired post-natal neurogenesis elicits unfavorable phenotypic deficits, it is predicted that pro-neurogenic chemical compounds should exhibit favorable therapeutic benefits.